This research will provide a basis for quantifying the relationship between contaminant exposure history and regulation of microbial biodegradative capabilities in natural porous media under flowing conditions representative of field scale plume contamination scenarios (low substrate concentrations and low levels of mixed electron acceptors [oxygen and nitrate]. Processes influences natural attenuation of four model compounds, representing three classes of priority contaminants (fuel hydrocarbons, chlorinated solvents, and polynuclear aromatic hydrocarbons) will be investigated. Well-characterized microbial/contaminant/porous media systems will be used to develop validated mathematical tools for the description of pollutant elution, transport, and microbial transformation. The experimental design is based on a series of integrated studies that range in scale from molecular (regulation of biodegradative pathways), to batch (kinetics of contaminant utilization), to bench-scale (flowing column studies), to field scale (stimulations of field conditions). Batch and column studies are proposed to: 1) evaluate the influence of substrate composition, porous media characteristics, and concentration exposure history of biodegradative gene expression under natural attenuation conditions; 2) investigate links between contaminant exposure history and regulation of biodegradative functions; 3) evaluate the influence of surface attachment on expression of biodegradative capabilities under natural attenuation conditions; and 4) develop state-of-the-art mathematical models to validate conceptual models, test hypotheses, and explore the implications of laboratory findings at the field scale. at the field scale. Knowledge gained from this Project pertaining to processes influencing microbial transformations will provide the foundation for improved Superfund site remediation at chlorinated solvent and hydrocarbon contaminated sites. In addition the work proposed here, in conjunction with Project 4e, will allow for development of approaches for assessment of 'alternative endpoint' clean- up standards under intrinsic and engineered remediation conditions.